This invention relates to apparatus for continuous manufacturing of butt-welded pipes, e.g., butt-welded steel pipes, by heating a skelp, bending it into a tubular shape, and butt-welding the opposite edges of the skelp to each other.
Generally, butt-welded pipes are manufactured by heating the edges of the skelp by passing the skelp through a heating furnace at a temperature of 1300.degree. C. suitable for butt-welding (hereinafter called the butt-welding temperature), bending the skelp into a tubular shape on molding rolls, butt-welding the opposite edges of the skelp to each other by means of butt-welding rolls, and then finishing the pipe by contraction rolling and other finishing processes. Such a method, however, is disadvantageous in that: the central portion of the skelp is heated as well as both the edges as it passes through the heating furnace, although the heating of the central portion is unnecessary and hence wasteful; both the edges of the skelp, which often are at somewhat different temperatures due to the construction of the heating furnace or differences of the thickness of the skelp itself, are butt-welded as they come from the furnace and the temperature difference causes a difference in deformation resistance which in turn makes the upset stress non-uniform; and the residual temperature difference between the seamed portion and the parent metal portion of the butt-welded pipe causes a deviation in the thickness in the vicinity of the seamed portion during the contraction process.
Heretofore, several remedies have been developed to overcome the above problems, but the results have not been completely effective. One method is disclosed in Japanese Patent Publication No. 14092/1968 and French Patent Application No. 936135. This method has been proposed to facilitate saving of energy, and in it the entire skelp is at first heated by passing it through the heating furnace at a temperature of about 1000.degree. C., which is lower than the butt-welding temperature. Then an inductor for induction heating is used to heat only the opposite edges up to the butt-welding temperature, so that the central portion of skelp is kept at a lower temperature than the butt-welding temperature and in a range suitable for carrying out the bending, contraction rolling, etc., without hinderance, thus achieving the desired energy saving.
Thus, the equipment for the above prior art method therefore can keep the central portion of skelp at a lower temperature than both edges, but that is all that is done. The temperature of the two edges of the skelp is not adjustable so that upset stress cannot be made uniform, and thickness deviation during the contraction rolling process after butt-welding, cannot be kept from increasing. This is because of the inductor used for heating the edges of the skelp in the above prior art method. FIG. 21 is a schematic exterior view of such an inductor. The inductor comprises an elongated copper bar 101, a pair of aligned short copper bars 102 and 103 disposed parallel to the copper bar 101 and spaced from the bar 101 a distance slightly greater than the width of a skelp 10, pairs of arcuate thick copper bands 104 and 104' bridged across the upper and lower surfaces of the front end of copper bar 101 and copper bar 102 and across the upper and lower surfaces of the rear end of copper bar 101 and copper bar 103, and a pair of terminal tongues 105 and 106 provided on the opposed ends of copper bars 102 and 103. The inductor thus has the copper bars 101, 102 and 103 extending parallel to the edges El and Er of skelp 10 and opposed to the edge faces of the skelp, and the skelp 10 travels between the thick copper bands 104 at one end of the inductor and between the thick copper bands 104' at the other end. During the travel of the skelp, high frequency current is supplied through terminals 105 and 106 to thereby generate induced currents on the surface of the moving skelp 10 as it cuts the magnetic flux generated around the inductor, thus selectively heating the two edges El and Er of the skelp 10.
However, the two edges El and Er of the skelp 10 discharged from the heating furnace and entering the upstream end of the inductor are not always at the same temperature due to the construction of the heating furnace or variations in the thickness of the skelp itself. Thus, even when the edges of the skelp are heated uniformly by the inductor, the edges will almost always differ in temperature, usually by about 5.degree. to 20.degree. C. Furthermore, the path of the skelp moving through the inductor may change slightly due to the natural movements of the skelp, so that the dimensions of the intervals between the copper bars 101, 102 and 103, and the two edges El and Er may vary, which increases the temperature difference between the two edges, so variations in the upset stress increase, resulting in poor quality of the finished product. Even though the spaces between the copper bars 101, 102 and 103, and the two edges El and Er of the skelp 10 are made smaller to try to increase the heating efficiency, the amount the space can be reduced is limited because of the danger of deforming the skelp if the edges El and Er contact the copper bars 101, 102 and 103. Hence, the heating efficiency must remain rather low and the opportunity for energy saving is limited. Moreover, an inductor of a particular size can handle only a limited number of different widths of skelp, so that inductors of various sizes must be available if many different widths of skelp are to be handled. Further, the aforesaid temperature difference between the two edges El and Er of the skelp 10 when it is discharged from the heating furnace must often be compensated for, but the inductor having the above-described construcion is unable to carry out such compensation. Hence, the above-described method has disadvantages due to the limitations in the apparatus used to carry it out.
Problems also exist due to the butt-welding rolls which are commonly used. The roll gap between vertically disposed butt-welding rolls has usually been set to be constant. There are, however, variations in the width of the skelp, and the butt-welding rolls are usually slightly eccentric so as to create variations in the pressure applied to the edges of the skelp. The roll gap may be made smaller in an effort to eliminate portions of reduced strength along the seamed portion of the continuous butt-welded pipe, but this causes the problem that bead and thickness variation along the seamed portion increases. Furthermore, in the above method in which the skelp is bent and the edges of skelp are heated to a higher temperature than the central portion thereof and then butt-welded, because the temperature difference between the edges and the central portion of the skelp is produced for the purpose of saving thermal energy, the deformation resistance in the central portion is larger than where the entire skelp is heated to the butt-welding temperature, so that the constant roll gap increases the pressure between the butt edges to thereby promote production of the bead along the seamed portion of the finished pipe and increase the thickness variation in the vicinity of the seamed portion, thus causing variations of strength along the seamed portion.